Best place to start...

So, I’ve decided to make a start on a simple gamemode. My problem doesn’t lie in my Lua coding capabilities but where to start.
So my question is, where is the best place to start on a gamemode?

The best place to start on a gamemode is to learn the Language.
http://www.facepunch.com/showthread.php?t=1337945

After that,

Amazing help man…

If you were meaning in terms that you know how to make a gamemode.

start with your spawning system. Wether thats team based
Job based. Free for all etc.